In the 3GPP Long Term Evolution (LTE) network and the further evolution of 3GPP Long Term Evolution (LTE-Advanced, LTEA) network, CoMP (Coordinated multiple point transmission and reception) is a very competitive option for improving the performance of the system.
Wherein, in the uplink inter-cell CoMP (also called as multiple BS coordinated receiving in the present specification), the core idea is: for a mobile terminal, the system determines a service BS and at least one assistant BS, the uplink signal sent by the mobile terminal will be received by the service BS and each assistant BS, then, the assistant BS forwards the uplink signal to the service BS, and the service BS merges the signal received directly by itself with the signals forwarded by the assistant BS, thus getting the mergence gain, which helps to improve the success rate of the signal reception and reduce the number of retransmissions.
FIG. 1 shows a typical multiple BS coordinated receiving network, wherein the BS 10 is the service BS for the mobile terminal 20, two adjacent BSs 11 and 12 of the BS 10 are assistant BSs. Of course, for other mobile terminals, the status of the three BSs may change, or other BSs may play the role of service BS or assistant BS.
Wherein, the assistant BS 11 and 12 forward the signals sent by mobile terminals through the wired backhaul link (eg, X2 interface) to the service BS 10. Wherein, the delay of the backhaul link is quite substantial, usually up to or even higher than 10 milliseconds (ms).
FIG. 2 shows the synchronous uplink HARQ process in the LTE standard, wherein, after a mobile terminal sends the uplink data, the waiting time is generally 4 ms, i.e., in FIG. 2, after sending the uplink service data of the uplink HARQ process 1 in sub-frame 4, it needs to receive the response in sub-frame 8, and, if the mobile terminal neither receives in the sub-frame 8 an ACK message which presents successful receiving, nor an NACK message which presents failed receiving, the mobile terminal will treat the uplink signal sent previously has not been correctly received by the receiver, therefore, will retransmit it later.
However, due to the multiple BS coordinated receiving network, the total delay produced by forwarding signals between BSs and merging signals is up to more than 10 milliseconds, which is contradictory to the response time limit of synchronous uplink HARQ process with the LTE standard.
In LTEA standard, it is not defined yet if the uplink HARQ process follows the proposal of LTE, wherein, if the LTEA standard using the same synchronous uplink HARQ technology with the LTE, then the technical problems will be the same with LTE, on the contrary, for LTEA people also need to provide an available proposal compatible with multiple BS coordinated receiving and synchronous uplink HARQ.